The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone, located in the mideastern United States. Situated 25 miles below the surface, it could be considered a "failed rift", because it does not split the North American plate.
The 150-mile long fault system, which extends into five states, stretches southward from Cairo, Illinois, through Hayti-Caruthersville and New Madrid, Missouri, through Blytheville, to Marked Tree, Arkansas. It also covers a part of Tennessee, near Reelfoot Lake, extending southeast into Dyersburg.
Structure
The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). The rift failed, but remained as a scar or zone of weakness. During the Mesozoic Era (about 200 million years ago), as the Atlantic Ocean was opening in the east, rifting was once again re-activated and intrusive igneous rocks were emplaced. But again the rifting failed and the continent remained intact, although with a significant zone of weakness. This rift is known as the Reelfoot Rift and coincides with the northernmost portion of the Mississippi embayment. Most of the seismicity is located from 5 to 25 km beneath the Earth's surface.
The red zones on the map above indicate the epicenter locations of hundreds of minor earthquakes recorded since the 1970s. Two trends are apparent. First is the general NE-SW trend paralleling the trend of the Reelfoot Rift. The second is the intense cross trend, NW-SE, that occurs just southwest of New Madrid. This second trend coincides with an intrusive igneous body which lies deeply buried beneath the sediments of the rift zone. Several other bodies of deeply buried intrusive rock are known to exist within the seismic zone. The depths of these igneous rock bodies closely corresponds to the depth of the seismic activity.
In New Madrid
The zone is named after the town of New Madrid, Missouri, the site of three of the most severe North American earthquakes in recorded history, including:
- December 16, 1811
- January 23, 1812
- February 7, 1812 was the strongest, with estimated magnitude exceeding 8
In the fault zone
The New Madrid fault zone lies within the central Mississippi River valley, extending from northeast Arkansas, through southeast Missouri, western Tennessee, western Kentucky to southern Illinois. Historically, this area has been the site of some of the largest earthquakes in North America. Between 1811 and 1812, four catastrophic earthquakes, with magnitude estimates greater than 7.0 on the Richter scale, occurred during a 3 month period. Hundreds of aftershocks followed over a period of several years. The largest earthquakes to have occurred since then were on January 4, 1843 and October 31, 1895 with magnitude estimates of 6.0 and 6.2 respectively. In addition to these events, seven events of magnitude >= 5.0 have occurred in the area. Instruments were installed in and around this area in 1974 to closely monitor seismic activity. Since then, more than 4000 earthquakes have been located, most of which are too small to be felt. On average one earthquake per year will be large enough to be felt in the area.
More quakes predicted
The potential for the recurrence of large earthquakes and their impact today on densely populated cities in and around the seismic zone has generated much research devoted to understanding earthquakes. By closely monitoring the earthquake activity, scientists can hope to understand their causes, recurrence rates, ground motion and disaster mitigation. The probability for an earthquake of magnitude 6.0 or greater is significant in the near future, with a 90% chance of such an earthquake by the year 2040.
Historical catastrophes
These catastrophic earthquakes occurred during a three-month period in December 1811 and early 1812. They caused changes in the course of the Mississippi River, which rolled backwards temporarily, and were felt as far away as New York City and Boston, Massachusetts, where churchbells rang. Large areas sank into the earth, fissures opened, lakes permanently drained, new lakes were formed, and forests were destroyed over an area of 150,000 acres (600 km²). Many houses at New Madrid were thrown down. "Houses, gardens, and fields were swallowed up" one source notes. But fatalities and damage were low, because the area was sparsely settled. Hundreds of aftershocks followed over a period of several years.
All three major quakes are generally believed to have exceeded 8.0 on the Richter Scale, and some seismologists believe the largest was 9.0 or larger. Scientists estimate that there is a 90% probability of a magnitude 6.0 to 7.0 quake on this fault system before 2040. Because of the unconsolidated sediments which are a major part of the underlying geology of the Mississippi embayment, large quakes here can affect as much as 20 times the land area of major quakes on the west coast.
Since 1812
The largest New Madrid seismic zone earthquakes to have occurred since then were on January 4, 1843 and October 31, 1895, with magnitude estimates of 6.0 and 6.2 respectively. The last major earthquake to occur in this region occurred in Charleston, Missouri, in 1895, and is estimated to have had a magnitude of 6.7 on the Richter Scale.
A request dated January 13th, 1814, by the Territorial Governor, William Clark, asked for Federal relief for the "inhabitants of New Madrid County". This was possibly the first example of a request for disaster relief, which would later become the job of the Federal Emergency Management Agency (FEMA).
See also
References
- USGS New Madrid http://wwwneic.cr.usgs.gov/neis/new_madrid/new_madrid.html
- U.S. Geologic Survey site discusses the New Madrid seismic zone. http://quake.wr.usgs.gov/prepare/factsheets/NewMadrid/
- Uncovering Hidden Hazards in the Mississippi Valley http://quake.wr.usgs.gov/prepare/factsheets/HiddenHazs/
- Links to eyewitness descriptions of the three 1811-12 earthquakes, and results of the earthquakes as seen in photographs of 1904. http://hsv.com/genlintr/newmadrd/
- Your Fault, My Fault, and the New Madrid Fault http://rockhoundingar.com/geology/fault.html
Last updated: 02-09-2005 04:45:48
Last updated: 05-03-2005 17:50:55
